In current common rail direct injection fuel systems, a low-pressure pump supplies fuel from a tank to a high-pressure pump, which in turn supplies the fuel to a common rail. A series of injectors (one for each engine cylinder) are connected to the common rail, which are cyclically piloted to inject part of the pressurized fuel in the common rail inside the respective cylinders. For correct combustion, it is important that the pressure level of the fuel inside the common rail be always kept at a desired value that, as a rule, varies as a function of the crank angle.
To keep the pressure value of the fuel inside the common rail equal to the desired value, it has been proposed to size the high-pressure pump to feed the common rail with a quantity of fuel in excess of the effective consumption under all operating conditions; an electromechanical pressure regulator is coupled with the common rail which maintains the pressure level of the fuel inside the common rail equal to the desired value by discharging excess fuel to a return line that re-injects this excess fuel upstream of the low-pressure pump. An injection system of this type has several drawbacks, as the high-pressure pump must be sized to supply the common rail with a slightly excessive quantity of fuel with respect to the maximum possible consumption; however, this condition of maximum possible consumption occurs quite rarely and in all the other remaining running conditions, the quantity of fuel supplied to the common rail by the high-pressure pump is much greater than the actual consumption and therefore a significant portion of this fuel must be discharged by the pressure regulator into the return line. The work carried out by the high-pressure pump to pump the fuel that is successively discharged by the pressure regulator is “useless” work, and therefore this injection system has very low energy efficiency. Furthermore, this injection system tends to overheat the fuel, because when the excess fuel is discharged by the pressure regulator into the return line, this fuel passes from a very high pressure to substantially ambient pressure and, due to the effect of this pressure jump, it heats up.
To resolve the above-described problems, a variable-flow high-pressure pump has been proposed that is able to supply the common rail with just the quantity of fuel necessary to keep the pressure of the fuel inside the common rail equal to the desired value.
For example, patent application EP0481964A1 describes a high-pressure pump equipped with an electromagnetic actuator able to vary the flow of the high-pressure pump moment by moment, by changing the instant of closure of an inlet valve on the high-pressure pump itself. In other words, the flow of the high-pressure pump is varied by changing the instant of closure of the inlet valve of the high-pressure pump itself; in particular, the flow is decreased by delaying the instant of closure of the inlet valve and increased by advancing the instant of closure of the inlet valve.
Another example of a variable-flow high-pressure pump is given by patent U.S. Pat. No. 6,116,870A1. The high-pressure pump described by U.S. Pat. No. 6,116,870A1 comprises a cylinder equipped with a piston having a reciprocating motion inside the cylinder, an intake line, a feed line connected to the common rail, an inlet valve able to allow the flow of fuel into the cylinder, a one-way discharge valve connected to the feed line and able to allow fuel flow only in output from the cylinder, and a regulator device connected to the inlet valve to keep the inlet valve open during a compression phase of the piston and therefore to allow fuel flow in output from the cylinder through the intake line. The inlet valve comprises a valve body that can move along the intake line and a valve seat, which is suitable for being engaged by the valve body to form a fluid-tight seal and is located at the end of the intake line opposite to the end communicating with the cylinder. The regulator device includes a control element, which is coupled to the valve body and is mobile between a passive position, in which it allows the valve body to engage the valve seat in a fluid-tight manner, and an active position, in which it does not allow the valve body to engage the valve seat in a fluid-tight manner; the control element is coupled to an electromagnetic actuator, which is able to move the control element between the passive position and the active position.
In cases of malfunctioning (mechanical, electrical or electronic) of the variable-flow high-pressure pump, the same variable-flow high-pressure pump could feed a much larger quantity of fuel than is necessary to the common rail, thus causing a rapid increase in fuel pressure inside the common rail; once this fault situation on the high-pressure pump is detected, the low-pressure pump is immediately switched off to interrupt the flow of fuel to the high-pressure pump and therefore block the uncontrolled rise of fuel pressure inside the common rail. However, switching off the low-pressure pump has a slightly delayed effect (equal to several pumping cycles in the high-pressure pump), and therefore without further actions of limitation, the fuel pressure inside the common rail could reach levels exceeding the maximum value physically supportable by the components of the injection system, with consequent rupture of these components and the discharge of fuel under high pressure into the engine compartment. To limit the maximum pressure of the fuel inside the common rail in cases of high-pressure pump malfunction, an electromechanical pressure regulator controlled by a control unit or, more frequently due to lower component costs, a mechanical pressure relief is always present in known injection systems.
The cases where the electromechanical pressure regulator or mechanical pressure relief is triggered are extremely rare; following such scarce usage, these components could have mechanical trouble due to moving mechanical parts sticking due to age and therefore might not be able to operate in a sufficiently efficient manner in case of need (i.e. in cases of malfunction in the high-pressure pump that causes a sudden increase in the fuel pressure inside the common rail).